1. Field of the Invention
The present invention relates to a magnetooptical recording medium allowing information recording and reproduction by irradiation with a laser beam, utilizing a magneto-optic effect, and methods for information recording and reproduction using the magnetooptical recording medium.
2. Related Background Art
In recent years, a magnetooptical recording medium has become a subject of attention in the field of a rewritable recording method of high recording density. In such a recording method, information or data is recorded in the recording medium by forming a magnetic domain in a magnetic film of the medium by means of thermal energy of a laser beam from a semiconductor laser, and information is read out from the medium, utilizing magneto-optic effect. The above-noted trend is based on need for a larger amount of recording capacity to be achieved by higher recording density of such a recording medium.
By the way, linear density of an optical disc, such as a magnetooptical recording medium, is largely dependent on the wavelength of a laser beam and the numerical aperture of an objective lens used in an optical system for reproducing information. When the wavelength .lambda. of a laser beam used in a reproducing optical system and the numerical aperture NA of an objective lens are determined, a bit or pit periodicity or pitch is defined as .lambda./2NA which is a minimum scale or limit of detection.
Track density of the optical disc is, on the other hand, chiefly limited by crosstalk. The crosstalk is largely dependent on a laser beam distribution or profile on a medium surface and is expressed as a function of .lambda./2NA, similar to the bit periodicity mentioned above. Thus, the wavelength of a laser beam must be shortened and the numerical aperture of an objective must be enlarged in order to increase recording density of the conventional optical disc.
However, there are limitations to improvement of the wavelength of a laser light and the numerical aperture of an objective. Techniques therefor have been developed, which improve the structure of a recording medium and a method of reading out data bits so that the linear recording density can be improved.
For example, Japanese Patent Laid-open No. 3-93058 discloses a method for improving the recording density of an optical disc. The process is performed in the following sequence. First, a recording medium, which comprises a readout layer and a recording layer, is prepared. After the direction of magnetization in the readout layer is oriented in a single direction prior to information reproduction, then information held in the recording layer is transferred to the readout layer. Thus, it becomes possible to reduce interference between codes at the time of information reproduction and to reproduce information recorded at a pitch below the diffraction limit of light.
However, the magnetooptical reproducing method of Japanese Patent Laid-open No. 3-93058 requires a step of aligning the magnetization direction of the readout layer in a predetermined direction, which is conducted before projection of a laser beam onto the readout layer. Thus, it is necessary to add a magnet for initializing the readout layer to a conventional apparatus. Due to such addition, problems arise, such as more complicated structure of a magnetooptical recording apparatus, difficulty of down-sizing, and higher cost of an apparatus.